An outboard motor often powers a watercraft. The motor includes a water propulsion device, such as a propeller, which is powered by an internal combustion engine. The engine has an output shaft that drives the water propulsion device.
The engine normally drives the propulsion device via a transmission. The transmission usually includes a dog clutch that moves between a pair of gears to couple a propulsion shaft to a drive shaft that the engine powers. The clutch moves between three drive positions: one position corresponds to a forward drive condition; another corresponds to a neutral drive condition; and another position corresponds to a reverse drive condition. An actuator moves the clutch between these positions, and forces the clutch into engagement with the driven gears to establish either the forward or reverse drive condition.
Cold starts of engine often are difficult due to poor combustibility during start-up. Some prior engines have addressed the problems associated with start-up operation by increasing the engine's idle speed until the engine has warmed to a sufficient degree (e.g., 40.degree. C.). Fuel and intake air volumes are often temporarily increased to elevate engine speed until the engine has warmed. Spark timing may also be advanced during start-up operations to a minimum advancing degree so as to obtain maximum torque in some applications (e.g., in automobiles).
This approach, however, can not be employed with an outboard motor. If the idle speed is too high, transmission engagement is difficult and not smooth. And if engine torque is set high during idle conditions, clutch engagement also is not smooth, even with low idle speed.
Outboard motors previously have controlled engine warm-up in the manner graphically represented in FIG. 9 (see the description of this figure below for explanation of the depicted curves). Immediately after the engine is started, the throttle valve is opened to decrease resistance to air intake as the engine begins to breath. The fuel volume added to the intake air to form the fuel charge is also increased immediately after the engine start (as represented by the straight section of curve a in FIG. 9). During this initial period, ignition timing also is advanced to 10.degree. before top dead center (BT10). The increase of the intake air volume and fuel volume is terminated after a certain period of time regardless of the coolant temperature. The ignition timing is delayed for every 15.degree. (as represented by line f) along with the increase of coolant temperature (as represented by line b). The engine speed (as represented by line e) is gradually reduced until the coolant temperature reaches a preset temperature (such as, for example, 40.degree. C.). The engine speed will slow to its normal idling speed (N.sub.ID) when the coolant temperature finally warms to 40.degree. C. and the ignition timing is delayed or retarded up to 10.degree. after top dead center (AT10).
This engine warm-up control method offers the advantage that the time period at which the engine speed is high is relatively short because the increase of intake air volume and fuel volume is limited only for a short period of time after the engine starts. Thus, the transmission can be engaged shortly after the engine start without encountering difficulties or significant mechanical shock.
Although theoretically this engine warm-up control method provides a suitable solution for the outboard motor application, it does not perform as well in practice. Because of errors in the manufacture the assembly process of the engine, each engine may have differing operating characteristics. Accordingly, the produced engine speed during engine warm-up may fluctuate from the design engine speed (as represented by line e). Some engines produce unexpectedly low or high speeds at a given coolant temperature and same advanced angles, as illustrated in FIG. 9, where phantom line c illustrates a higher engine speed relative to a design engine speed (line e), and phantom line d represents the lower engine speed relative to the design engine speed (line e). As a result, the engine may stall or may idle for a longer period of time at an elevated speed, which makes transmission shifting difficult until the engine warms.